[Python] Why does pandas.Series allow a list as an index label (e.g. [['A'], 'B', 'C'])?

I accidentally created a pandas.Series using a list as part of the index, and to my surprise, it worked without any error:

import pandas as pd

series = pd.Series([1, 2, 3], index=[['A'], 'B', 'C'])
print(series)
print(type(list(series.index)[0])) # outputs: <class 'list'>


This creates a Series object where the first index value is a Python list (['A']), , which surprised me because I assumed that all index labels in a pandas Series must be hashable types (like strings, numbers, or tuples). Lists, being mutable, are not hashable — so I expected an error.

However, I can still print and access the Series — at least partially — without an immediate error.

When I try this:

series.loc[['A']]


I get TypeError: unhashable type: 'list'

Questions:

• Why does pandas allow a list as an index label in the first place?
• Is this behavior intentional or just a side effect of how Index objects are handled internally?

I'm using pandas version 2.2.3.

Any insights or references would be greatly appreciated!

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